1. Field of the Invention
The invention relates to a system and a method for providing sound to at least one user, wherein audio signals from an audio signal source, such as a microphone for capturing a speaker's voice, are transmitted via a wireless link to a receiver unit, such as an audio receiver for a hearing aid, from where the audio signals are supplied to means for stimulating the hearing of the user, such as a hearing aid loudspeaker.
2. Description of Related Art
Presently, in such systems, the wireless audio link usually is an FM (frequency modulation) radio link. According to a typical application of such wireless audio systems, the receiver unit is connected to or integrated into a hearing instrument, such as a hearing aid, with the transmitted audio signals being mixed with audio signals captured by the microphone of the hearing instrument prior to being reproduced by the output transducer of the hearing instrument. The benefit of such systems is that the microphone of the hearing instrument can be supplemented or replaced by a remote microphone which produces audio signals which are transmitted wirelessly to the FM receiver, and thus, to the hearing instrument. In particular, FM systems have been standard equipment for children with hearing loss in educational settings for many years. Their merit lies in the fact that a microphone placed a few centimeters from the mouth of a person speaking receives speech at a much higher level than one placed several feet away. This increase in speech level corresponds to an increase in signal-to-noise ratio (SNR) due to the direct wireless connection to the listener's amplification system. The resulting improvements of signal level and SNR in the listener's ear are recognized as the primary benefits of FM radio systems, as hearing-impaired individuals are at a significant disadvantage when processing signals with a poor acoustical SNR.
A typical application of such wireless audio systems is at school, wherein the teacher uses a wireless microphone for transmitting the captured audio signals via the transmission unit to receiver units worn by the students. Since the receiver units and the respective hearing aids are usually owned by the students, the receiver units may be of different types within a class.
Another typical application of wireless audio systems is the case in which the transmission unit is designed as an assistive listening device. In this case, the transmission unit may include a wireless microphone for capturing ambient sound, in particular from a speaker close to the user, and/or a gateway to an external audio device, such as a mobile phone; here the transmission unit usually only serves to supply wireless audio signals to the receiver unit(s) worn by the user.
Examples of analog wireless FM systems particularly suited for school applications are described, for example, in European Patent Application EP 1 863 320 A1 and International Patent Application Publication WO 2008/138365 A1. According to these systems, the wireless link not only serves to transmit audio signals captured by the wireless microphone, but in addition, also serves to transmit control data obtained from analyzing the audio signals in the transmission unit to the receiver unit(s), with such control data being used in the receiver unit to adjust, for example, the gain applied to the received audio signals according to the prevailing ambient noise and the issue of whether the speaker is presently speaking or not.
In applications where the receiver unit is part of or connected to a hearing aid, transmission is usually carried out by using analog FM technology in the 200 MHz frequency band. In recent systems, the analog FM transmission technology is replaced using digital modulation techniques for audio signal transmission. An example of such digital system is available from the company Comfort Audio AB, 30105 Halmstad, Sweden under the COMFORT DIGISYSTEM® trademark.
A specific example of an analog wireless FM system particularly suited for school applications is described in International Patent Application Publication WO 2008/074350 A1, wherein the system consists of a plurality of transmission units comprising a microphone and a plurality of analog FM receiver units and wherein only one of the transmission units has an analog audio signal transmitter, while each of the transmission units is provided with a digital transceiver in order to realize an assistive digital link for enabling communication between the transmission units. The assistive digital link also serves to transmit audio signals captured by a transmission unit not having the analog transmitter to the transmission unit having the analog transmitter from where the audio signals are transmitted via the analog FM link to the receiver units.
A presentation by S. Shellhammer “SCORT—An Alternative to the Bluetooth SCO Link for Voice Operation in an Interference Environment” document IEEE 802.15-01/145r1, March 2001, and of the IEEE P802.15 Working Group for Wireless Personal Area Networks, relate to a proposed alternative for the Bluetooth SCO link for operation in an interference environment, wherein it is proposed to use, in a bi-directional point-to-point link (i.e., full duplex link) for voice transmission, repeated transmission of the same audio packet without involving a receipt acknowledgement by the receiving device.
U.S. Patent Application Publication 2007/0009124 A1 and corresponding U.S. Pat. No. 7,778,432 B2 relate to a wireless network for communication of binaural hearing aids with other devices, such as a mobile phone, using slow frequency hopping, wherein each data packet is transmitted in a separate slot of a TDMA frame, with each slot being associated to a different transmission frequency, wherein the hopping sequence is calculated using the ID of the master device, the slot number and the frame number. A link management package is sent from the master device to the slave devices in the first slot of each frame. The system may be operated in a broadcast mode. Each receiver is turned on only during the transmission during time slots associated to the respective receiver. The system has two acquisition modes for synchronization, with two different handshake protocols. Eight LMP messages are transmitted in every frame during initial acquisition, and one LMP message is transmitted in every frame once a network is established. Handshake, i.e., bi-directional message exchange, is needed both for initial acquisition and acquisition into the established network. During acquisition, only a reduced number of acquisition channels is used, with the frequency hopping scheme being applied to these acquisition channels. The system operates in the 2.4 GHz ISM band. A similar system is known from U.S. Patent Application Publication 2009/0245551 A1 and corresponding U.S. Pat. No. 8,229,146 B2.
U.S. Pat. No. 7,532,610 B2 relates to an adaptive frequency hopping scheme, wherein bad frequencies are empirically excluded from the frequency range used by the frequency hopping algorithm.
International Patent Application Publication WO 2008/135975 A2 relates to a communication network, wherein the receiver wakes up for listening to the preamble of a data packet and goes to sleep again, if no valid preamble is received.
U.S. Patent Application Publication 2006/0067550 A1 relates to a hearing aid system comprising at least three hearing aids between which a wireless communication network is established using the Bluetooth standard, wherein one of the hearing aids is used for receiving signals from another one of the hearing aids, amplifying the signals and forwarding it to the third hearing aid.
U.S. Patent Application Publication 2007/0086601 A1 relates to a system comprising a transmission unit with a microphone for transmitting a speaker's voice to a plurality of hearing aids via a wireless digital link, which may be unidirectional or bi-directional and which may be used for transmitting both audio data and control data to the hearing aids.
U.S. Pat. No. 7,529,565 B2 relates to a hearing aid comprising a transceiver for communication with an external device, wherein a wireless communication protocol including a transmission protocol, link protocol, extended protocol, data protocol and audio protocol is used. The transmission protocol is adapted to control transceiver operations to provide half duplex communications over a single channel, and the link protocol is adapted to implement a packet transmission process to account for frame collisions on the channel.
U.S. Pat. No. 7,606,291 B2 relates to a two-way push-to-talk radio device using frequency hopping.
European Patent Application EP 1 560 383 A2 relates to a Bluetooth system, wherein the slave device, in a park mode or in a sniff mode, periodically wakes up to listen to transmission from the master and to re-synchronize its clock offset.
A problem which may occur in wireless radio links is multi-path fading. Due to reflections of the radio waves on the ground or on other objects, destructive interference may occur and cause a reduction in the received signal amplitude, depending on the position of the transmitter. When a plurality of antennas are used, it is likely that, if the signal is suffering from multi-path fading at the location of one of the antennas, there will be no multipath fading at one of the other antennas. The condition for this is that the distance between the two antennas is large compared to the wavelength of the radio carrier, i.e., at least on the order of magnitude of the wavelength. One known approach to utilize this antenna diversity effect is known as “switched diversity”, wherein the signal power at two spaced apart antennas is measured and the input of the radio demodulator is switched to that antenna which presently provides for the higher signal power, i.e., that antenna which presently suffers less from multi-path fading.
Another known technique using spaced apart antennas is known as “combinational diversity”, wherein the received signal is simultaneously sampled using multiple antennas and demodulators. These techniques use low level physical algorithms to minimize the bit error rate, see also MIMO (Multiple Input Multiple Output) systems. Combinational diversity has a drawback in that it requires specialized hardware.
Switched diversity (selection diversity) is not optimal for packet communication when packets are small. When the channel conditions are changing rapidly, for example, due to movement of the transmission device, switched diversity can be too slow in reaction. This is particularly important when a packet radio communication system is used, wherein the packets are of short duration, for example, 100 to 200 μs. Moreover, switched diversity requires specialized hardware.
U.S. Patent Application Publication 2002/0183087 A1 and corresponding U.S. Pat. No. 7,103,340 B2 relates to a Bluetooth link for a mobile phone using two parallel antennas/transceivers, wherein each data packet is sent once and wherein for a sequence of packets, usually for the next 8 packets, a certain one of the antennas is selected according to previous channel quality measurements as a function of frequency. For each packet of the sequence one of the antennas is selected depending on the respective frequency at which the packet is to be transmitted, wherein the frequency is determined by a frequency hopping sequence.
U.S. Patent Application Publication 2006/0148433 A1 and corresponding U.S. Pat. No. 7,489,913 B2 relates to a wireless link between a mobile phone and a base station of the mobile network, wherein two receivers are used in parallel for achieving diversity if the coverage is poor.
Canadian Patent CA 2 286 522 C relates to a diversity radio reception method, wherein two data packets received in parallel by two receivers are compared and, if they differ from each other, the more reliable one is selected for further processing.
In “Effect of Antenna Placement and Diversity on Vehicular Network Communications” by S. Kaul, K. Ramachandran, P. Shankar, S. Oh, M. Gruteser, I. Seskar, T. Nadeem, 4th Annual IEEE Communications Society Conference on Sensor, Mesh and Ad Hoc Communications and Networks, 2007, SECON '07, pp. 112-121, a packet level diversity approach is described, wherein in a vehicle-to-vehicle link using roof- and in-vehicle-mounted omni-directional antennas and IEEE 802.11a radios operating in the 5 GHz band a packet level selection diversity scheme using multiple antennas and radios is utilized to improve performance not only in a fading channel but also in line-of-sight conditions.
A similar approach is used in “Packet-Level Diversity—From Theory to Practice: An 802.11-based Experimental Investigation” by E. Vergetis et al., MobiCom '06 (see also http://repository.upenn.edu/ese_papers/194), wherein a packet level diversity scheme is applied to a wireless data link between a laptop computer and an access point.